1. Field
The following description relates to a method of a sensor network, and more particularly to a data transmission method in a sensor network.
2. Description of the Related Art
Wireless sensor networks (WSN) are formed to comply with the Institute of Electrical and Electronics Engineers (IEEE) 802.15.4 standards. Although a sensor node forming the sensor network should be in accordance with schemes of a physical (PHY) layer and a media access control layer, which are specified by IEEE 802.15.4, sensor nodes have been recently manufactured according to various medium access control (MAC) technologies which do not conform with the standards. Generally, sensor nodes are configured in accordance with appropriate MAC technologies including IEEE 802.15.4, simple MAC (S-MAC) and Berkely MAC (B-MAC), a PHY layer is formed in strict compliance with IEEE 802.15.4 standards, and MAC technologies for a sensor network are implemented to be suitable for a low data transfer rate (between about 40 kbps and 250 kbps) which is specified in the PHY layer.
Since conventionally it was assumed that the amount of information obtained by sensors in a sensor network is small, the sensors were regarded suitable to transmit and receive data even at a low data transfer rate in the sensor network. However, recently there have been increasing attempts to transmit image data by the use of a camera device mounted on a sensor node, and thus the need for large data transmission rates has arisen.
When large data transmission rates are required, conventionally, the large amount of data is segmented and processed in a higher layer of MAC. In a conventional MAC layer, as many data frames as the number of data segments are processed without information of the data segment.
Generally, a usual image file has a size exceeding 20 Kbytes. On the assumption that MAC data of 20 Kbytes with its MAX_FRAME_LENGTH of 128 bytes is transmitted by use of a sensor node using 915 MHz band at data rate of 40 kbps, the sensor node should transmit 100 bytes of data 200 times in order to transmit the whole MAC data since MAC payload, i.e. the amount of data size which can be transmitted at once considering the size of MAC header, is 100 bytes. Additionally, when a reception time of an acknowledgement (ACK) frame at each time of transmission is considered, the ACK frame indicating whether the data is received by a receiving node, the total size of data is (128 bytes (MAC data including MAC header)+5 bytes (ACK))×200 (frequency)=26,600 bytes. Since the data rate of the sensor node is 40 Kbits/sec, the total time spent on transmitting packets of 26,600 bytes is 26,600×8 bits/40 Kbits=about 5.32 seconds.
The above calculation is simply conducted without considering the fact that the sensor node waits for macAckWaitDuration (three times more than a normal data transmission time) when the sensor node does not receive a subsequent ACK frame while repeating transmission of following data after confirming receipt of an ACK frame. Accordingly, the total transmission time is generally increased. In addition, since the above calculation does not take into account the processing time for the ACK frame, the actual total transmission time is much longer, and thus it may take several tens of seconds or more than one minute.
For example, if the above large data transmission technique using a MAC high layer for processing the large data is employed in a security system which monitors the opening and closing of a door to detect an intruder, and if an intruder is present, photographs the intruder, and transmits an image of the intruder to a control center, then since it takes several tens of seconds to transfer the image data from one sensor node to another in a sensor network, it may take more than several tens of seconds, i.e., several minutes for the image data to reach the control center.